题目

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

原题地址

解题思路

验证一个字符串是否是回文字符串。首先看看wiki上对于回文的解释：回文，亦称回环，是正读反读都能读通的句子，亦有将文字排列成圆圈者，Famous examples include "Amore, Roma", "A man, a plan, a canal: Panama" and "No ‘x‘ in ‘Nixon‘"。
判读一个字符串是否是回文，一种方法可以将字符串倒置然后和原字符串进行比较。这里采用一种类似字符串翻转的方法，通过从前后两个方向来比较判断是否是回文。本题中的有效字符是字母和数字，并且忽略字母大小写。

代码如下

class Solution {
public:
    bool isPalindrome(string s) {
        int n = s.size();
        int i=0, j=n-1;
        while(i<j){
            if( !isalpha(s[i]) ) {
                ++i;
                continue;
            }
            if( !isalpha(s[j]) ) {
                --j;
                continue;
            }
            if(s[i] != s[j]) return false;
            ++i,--j;
        }
        return true;
    }
    //判断是否是字母数字，如果是大写字母则将其转化为小写字母
    bool isalpha(char &c){
        if((c>='A'&&c<='Z')){
            c = c-'A'+'a';
            return true;
        }
        return (c>='a'&&c<='z') || (c>='0'&&c<='9');
    }
};